1. Field of the Invention
The present invention relates to an electric disc brake apparatus for producing braking force using the torque of a motor, and more particularly to an electric disc brake apparatus with an added function as a parking brake.
2. Description of the Related Art
There are currently electric disc brake apparatuses with a caliper comprising a piston, a motor, and a rotation-linear movement conversion mechanism for converting the rotation of the motor into linear movement and transmitting this linear movement to the piston. In such an electric disc brake apparatus, the piston is moved in accordance with the rotation of a rotor of the motor, whereby a brake pad is pressed against a disc rotor to produce braking force. Further, such an electric disc brake apparatus normally has a sensor for detecting the pressing force or stroke of a brake pedal depressed by an operator, and controls the rotation (rotational angle) of the electric motor in accordance with the detected value to thereby obtain the desired braking force.
In recent times, various attempts have been made to enhance the utility value of this type of electric disc brake apparatus by adding a parking brake function thereto.
In order to use an electric disc brake, which possesses reversibility against the reactive force from the brake pad, as a parking brake, the piston must be fixed by some type of means.
For example, in the electric caliper which converts the rotational motion of the motor into linear movement, the rotor of the motor is locked by a solenoid actuator (to be referred to as “solenoid” below). To use the apparatus as a parking brake, the solenoid must be locked into a non-energized state, and for this purpose (1) a mechanism which sets the solenoid in an energized state such that the lock is released during normal braking, and sets the solenoid in a non-energized state such that the lock is activated during parking braking, (2) a mechanism which uses a solenoid comprising a latch mechanism such that during normal braking, the solenoid is moved in a releasing direction and thus set in a momentary energized state such that the lock is released, and during parking braking, the solenoid is moved in a locking direction and thus set in a momentary energized state such that the lock is activated, and so on are used.
An example of an electric disc brake apparatus having a parking brake function as described above is illustrated in U.S. Unexamined Patent Application Publication US2003/0066719A1 (or the corresponding German Unexamined Patent Application Publication (Offenlegungsschrift) DE10233673A1).
It is desirable to understand clearly when the parking brake is operative and when it is inoperative. In the prior art described above, however, it is difficult to know when the parking brake is operative, and hence the apparatus in the prior art cannot satisfy this requirement suitably.